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The structure and dynamics of 1,3,5-cycloheptatriene and 1,3-cycloheptadiene radical cations in low-temperature matrices. An ESR investigation
Chemical Physics North-Holland
160
( 1992) 42 l-426
The structure and dynamics of 1,3,5-cycloheptatriene and 1,3_cycloheptadiene radical cations in low-temperature matrices. An ESR investigation Yoshihiro Yoshihisa
Hiroshi
Kubozono, Tatsuya Matsuda. Yasuhiko
Nakamura
and Taku
Miyamoto, Gondo
Makoto
Aoyagi,
Ata’,
Matsuo
Department of Chenucal Suence and Technology. Faculty ofEngrneenng, Hakozalci, Hlgashlku. Fukuoka 812, Japan Received
Masafumi
Kyushu Unwers@v.
18April I99 I, m final form 10 October I99 I
ESR spectra of the radxal species dewed from “‘Co y-ray lrradlatlon of 1.3.5.cycloheptatrlene ( 1.3.5-CYT) and 1,3-cycloheptadlenc ( I.3-CYD) m halocarbon matrlces have been studied m the temperature range 70-I 30 K. Rmg mvewon across the molecular plane occurs m the radical catlon 1.3.5~CYT+ m CCI,CF,. the actlvatlon energy bemg 1.7 kcal/mol. Above 90 K. I .3,5-CYT+ IS deprotonated thermally m CC12FCCIF:. No dynamlcal effect has been observed for I.3-CYD+
1. Introduction ESR spectroscopic studies on the radical species radiolytically derived from cyclic hydrocarbons in halocarbon matrices have been done by many investigators [ l-l 21. Shida and co-workers studied the ring puckering of the tetrahydrofuran radical cation [ 71, while Iwasaki et al. investigated the dynamical properties of the cyclopropane and cyclohexane radical cations [ 8 1. Lund and co-workers investigated the ring puckering (pseudorotation) and inversion of the cyclopentane radical cation [ 91 and the interconversion between the two twisted CZ conformations of the cyclopentyl radical [ IO]. Recently. Shiotani and co-workers reported on the mirror inversion between two energetically equivalent structures of the radical cations of methyl-substituted cyclohexanes [ 11.121. We here report the dynamical behavior of 1.3,5CYT+ in CCl,CF3 and other halocarbon matrices in the temperature range 70- 130 K. as well as the ther’ Present address: SONY Research
Center,
Yokohama
740,
05.00 0 1992 Elsewer Sc!ence Pubhshers
B.V
ma1 reaction of it in CClZFCClF2. The molecular structure and dynamical behavior of 1.3-CYD+’ will also be discussed.
2. Experimental Commercially available I ,3,5-CYT, 1.3-CYD (Aldrich) and the halocarbons CCIJF, CC13CF3, CCl,FCClF,, and CCIZFCCllF (Tokyo Kasei) were used as received. Sample solutions of about 0.1-0.2 ~01% concentration were prepared under vacuum. The radical species were generated by ‘j°Co y-ray irradiation at 77 K at a total dose of 0.7 Mrad (1 Mrad= 10” J kg-‘). ESR measurements were made with a JEOL RE3X ESR spectrometer combined with a temperature controller (Oxford Instruments E900 helium flow cryostat ).
Japan 0301-0104/92/$
All rlghtr reserved
Y. Kubozono et al. / 1.3.5~CYTand
422
I,3-CYD radical cations
3. Results and discussion 3.1. The radlcalspecles derivedflow 1,3,5CYT The ESR spectrum of 1,3.5-CYT+’ in CCl,CF, observed at 70 K is shown in fig. la to consist of three main groups of lines. Fig 1b shows that the spectrum is well simulated with the hyperfine coupling (hfc) constants of the axial P-proton aHa =57.0 G, equatorial P-proton a”“( eq) ~46.7 G, four cr-protons aH’Z4.h(4H)=5.8 G, aH’5(2H)=0 G, and the Gaussian line width AH= 5.0 G; the terms “axial” and “equatorial” are used exaggeratedly for the convenience of discussion. The results show that 1,3,5CYT+‘ has a fixed nonplanar structure in CClXCF3 at 70 K. The P-proton couplings arise from hyperconjugation. since the SOMO of the planar structure is of n-type.
Fig. I. ESR spectrum of 1.3.5-CYT+- m CC13CF, observed at 70 K (a) and the stimulated spectrum wth the ESR parameters gwen m the text (b).
E/eV
hfcc/G _
/----.
-995.2
‘OO.O I /
.‘. -996.0
\
1,3,5-CYT
OL 50.0 I
0
Fig. 2 shows the UHF-INDO hfc constants and UHF-AM 1 total energy calculated for the geometries optimized using the single-point UHF-AM 1 calculation [ 131, as functions of r3?under the constrained C, symmetry of 0, = 0’. 8, and o2 are the dihedral angle between the planes C,-C,-C,-C, and C,-C,-C,-C,, and that between the planes C,-C,-C, and C,-C2C5-C,, respectively. The initial optimization was done by allowing both 8, and @Ito vary. In 1,3,5-CYT, 0,=40.5&2’ and &=36.5&2” as determined by electron diffraction [ 141. The 19~value of 0” for 1.3,5CYT+.. associated with the UHF-AM 1 energy minimum, shows that the carbon framework, C,-C&3-
<
_,OO~_._t_-*_ 0
IO
20
*&i
-997.0
*
:&j
30
40
50
02/ O Fg. 2. The UHF-INDO hfc constants and UHF-AM I total energles of 1.3,5-CYT+ 0: a”@(ah), 0: aHfl(eq). *: uHIb, w: uHlr. 0: a”jn, *: total energy.
C&Zs-C,, is more nearly planar in 1,3,5-CYT+’ than in 1,3,5-CYT. The UHF-INDO hfc constants a”fl( ax) and a “fl( eq) are much larger than the experimental
Y. Kubozono et al. / 1.3.5~CYTand
423
I,3-CYD radtcal cations
ones, and from the experimental and theoretical ratios of a “D(ax) to a “fl( eq) 0: is estimated to be 5 ‘. The theoretical uH1,h and Q”~.~ are practically independent of 19:and are close to each other. The UHFAM 1 total energy is minimal at &=O”. In view of these results. 1.3,5-CYT”. is slightly nonplanar in CCl,CF,. Fig. 3 shows the ESR spectra of 1.3,~~CYT+’ in CCl$ZF, observed in the range 90-120 K. every spectrum consisting of three main groups of quintet lines. The typical line width alternation occurs at 90 K and with increasing temperature the alternation diminishes gradually. This is attributable to the ring inversion illustrated below.
108 K
114 K
120 K
1,3,5-CYT+’ disappeared above 125 K before reaching the fast-motional limit, where the intensity ratio among the three main groups of lines is 1:2: 1. The ring inversion exchanges the axial and equatorial /3protons, while it affects the other protons only little. Thus, two-site jump simulatron was done with the modified Bloch equation, as fig. 4 shows. The jump rate was determined at every 2 K in the range 90- 121 K. The Arrhenius plot is linear above 104 K, as fig. 5 shows, and a least-squares fit gives an activation energy of 1.7 kcal/mol. This value is comparable with those for the ring inversions of the radical cations of tetrahydrofuran, cyclopentane and pyrolidine in halocarbon matrices [ 7,9,15 1. In the range 90-l 02 K a much smaller activation energy of 0.4 kcal/mol was obtained. Concerning the bent Arrhenius plot, at present we can only suggest a matrix phase transition around 104 K. The mechanism involved deserves to be investigated in detail. Shida and co-workers previously found a similar Arrhenius plot for the internal rotation of the methyl group in the dimethylether
Fig. 3 ESR spectra of 1,3.5-CYT’ temperature range 90- 120 K.
m CCI,CF,
observed
in the
radical cation in CC13F and attributed the smaller activation energy to tunneling [ 16 1. A spectral change due to dynamical motion was not clearly observed in the other halocarbon matrices, CC13F, CC12FCClF,, and CC12FCC12F. The tripletquintet ESR spectrum of 1,3,5-CYT+’ in CC13F observed at 90 K is the same as those at 15 and 130 K, except for the increasing line broadening with decreasing temperature. In other words, the effect of dynamical motion is not appreciable, indicating that the difference between the hfc constants of the axial and equatorial P-protons is small as compared with the line width of about 5.0 G. Thus, 1,3,5-CYT+. is more nearly planar in CCl,F than in CC13CF3. Fig. 6 shows the ESR spectrum observed at 80 K and that at 90 K for the radical species derived from I ,3,5-CYT in CClzFCClF2. The former spectrum can
424
Y. Kubozono et al. / I,J,S-CYTand
kSj.56
x10*s-’
1.3~CYD radual cations
Fig.
Arrhemus
plot of the rate constant,
In k versus
I /T.
Fig. 4. Smulatlon ofthe ESR spectra of 1,3,5-CYT+’ m CCIjCFj observed tn the temperature range 90-120 K For details, see text.
be attributed to 1,3.5-CYT+‘, while the latter is assigned to a deprotonation-type neutral radical, C,H;. Fig. 6c shows that the spectrum is well simulated with the reasonable hfc constant u” (7H) ~4.2 G for the seven cr-protons and the line width Al?= 3.3 G [ I 7 1. The 80 K spectrum of I.35CYT+ ’ is probably a little distorted by the presence of C,H;. The occurrence of thermal deprotonation reflects directly the softness of the matrix.
Fig. 6. ESR spectra. observed at 80 K (a) and 90 K (b), of the radlcak generated by the exposure of 1,3,5-CYT to “Co y-rays m CCLFCCIF, at 77 K. The respective spectra are asslgned to 1.3.5CYT+ and the deprotonatlon-type radical, C,H,. The spectrum (c) snnulated with the ESR parameters given m the text refers to . the ooserved spectrum (b)
Y. Kubozono et al. / 1.3.5CYTand
425
1.3~CYD radrcal cations
3.2. I,3-CYD+’ Fig. 7a shows the ESR spectrum of 1,3-CYD+’ in CC13CFj observed at 80 K. Fig. 7b shows that the spectrum is well simulated with the hfc constants of the two axial/?-protons a”fl(ax) =28.5 G, two equatorial D-protons aHo = 10.0 G. two a-protons aH~~5(2H)=10.0G,aH3~4(2H)=OG,aH~(2H)=OG and the line width AH=5.4 G. The observed spectrum gives no evidence for dynamical averaging even at high temperatures ( > 100 K), indicating that in the matrix the structure of 1,3-CYD+’ is practically fixed in the range 40-l 10 K; the radical disappears around 120 K. Fig. 8 shows the UHF-ND0 hfc constants and UHF-AM 1 total energy as functions of 0,. As in fig. 2, 8, is fixed at O”, where the UHF-AM1 energy is minimal in the initial optimization allowing both 0, and 8: to vary. Fig. 7. ESR spectrum of 1.3-CYD+‘ m Ccl&F3 observed K (a) and the simulated spectrum with the ESR parameters m the text (b)
at 80 gwen
Yax. Yeq. hfdG
E/eV
Oeq.
1,3-CYD
Fig. 8 The UHF-INDO hfc constants and UHF-AM1 total energles of 1,3-CYD+ A: a”#(ax), A: aHB(eq), 0: aHy(ax). 0: a”Y(eq). n : a H25 , 0: a”,+, *: total energy.
Incidentally, 8, is 0” in I,3-CYD [ 181. Just as in 1,3,5-CYT+‘, the theoretical aHg values are overestimated, and from comparison of the experimental and theoretical ratios of aHo to the aHB(eq) 8, is assessed to fall in the range 15-25 O. The other theoretical hfc constants practically agree with the observed ones in this & range. The total energy reaches a minimum at e2=00, so that tY2range can be regarded as
426
Y Kubozono et al. / 1,3,5-CYTand
consistent taking account of the matrix effect. The lower degree of planarity in 1,3-CYD+’ than in 1,3,5CYT+’ may be associated with the resistivity against ring inversion in the former. With increasing 6, the UHF-AM 1 total energy increases more steeply in 1,3CYD+’ than in 1.3.5-CYT+‘, as fig. 8 shows, which also tends to prevent ring inversion. On the other hand, 1.3-CYD+’ is more nearly planar than 1,3-CYD for which &= 72.8’ as determined by electron diffraction [ 18 1. At 90 K the spectrum for the species derived from 1.3-CYD in CC12FCC1F2 is ascribed to 1,3-CYD+‘, but it turns to be complex with increasing temperature. The spectrum in CC13F did not show any temperature dependence up to 150 K, above which 1.3-CYD+‘ disappeared.
4. Concluding remarks In the radical species studied, the 19~values oftotalenergy minima deviate slightly from 0”. probably reflecting the matrix effects. The fairly higher degree of planarity of 1.3,5-CYT+. as compared to that of 1,3CYD+’ may be associated with the occurrence of dynamical motion in the former.
Acknowledgement The authors are grateful to Professor Yusaku Ikegami of Tohoku University and Professor Masaru Shiotani of Hiroshima University for their helpful discussions. The authors also thank Dr. Hiroshi Sekiya of Kyushu University and Dr. Noriyuki Kishii of SONY Research Center for their valuable suggestions. The MOPAC Ver. 5.0 was used for the UHFAM 1 calculations [ 13,19,20]. MO calculations were
1.3~CYD radlcalcatlons
carried out on a FACOM M-780/20 computer Computer Center of Kyushu University.
at the
References [ 11T. Shtda, Y. Egawa and H. Kubodera.
J. Chem. Phys. 73 (1980) 5963 [2] M Tabata and A. Lund. Chem. Phys. 75 (1983) 379. [3] T. Shtda and Y. Takemura. Radtation Phys. Chem. 21 (1983) 157. [4] K. Ushida, T. Shida, M. Iwasakt. K. Tortyama and K. Nunome, J. Am. Chem. Sot. 105 (1983) 5496. [ 51 K Tortyama. K. Nunome and M. Iwasakt. J. Chem. Sot Chem. Commun. (1984) 143. [6] K. Ohta, M. Shtotant, J. Sohma, A. Hasegawa and M.C.R. Symons, Chem Phys. Letters 136 (1987) 465. [ 71 H. Kubodera. T. Shtda and K. Shimokoshi, J. Phys. Chem. 85 (1981) 2583. [8] M. Iwasakt, K. Tortyama and K. Nunome. Faraday Dtscussions Chem. Sot. 78 ( 1984) 19. [9] L. SJoqvtst. A. Lund and J. Maruani. Chem. Phys. 125 (1988) 293. [ lo] L SJoqvtst, M. Lrndgren and 4. Lund, Chem. Phys. Letters 156 (1989) 323. [ 111 L. SJoqvtst. M. Lmdgren, A. Lund and M Shtotani, J. Chem. Sot. Faraday Trans. 86 ( 1990) 3377. [ 121 M. Lmdgren and M. Shiotam. m: Radical Ionic Systems, eds. A. Lund and M. Shtotani (Kluwer Academrc Publishers, Dordrecht, 1991) p. 125. [ 131 M.J.S. Dewar, E.G. Zoebisch, E.F. Healy and J.J.P. Stewart, J. Am. Chem. Sot. 107 (1985) 3902. [ 141 M. Traetteberg, J. Am. Chem. Sot. 86 (1964) 4265. [ 151 M. Shtotani. L. Sjoqvist. A. Lund, S. Lunnell. L. Eriksson and M.-B. Huang, J. Phys. Chem., in press. [ 161 M Matsushtta, T. Momose and T. Shida, J. Chem. Phys. 92 (1990) 4749. [ 171 D.E. Wood and H.M. McConnell. J. Chem. Phys. 37 (1962) 1150.
[ 181J.F. Chiang and S.H. Bauer, J. Am. Chem. Sot. 88 ( 1966) 420
[ 191J.J.P. Stewart. QCPE Bull. 9 (1989) 10. [ 701T. Htrano. JCPE Newsletter 1 ( 1989) 10
160
( 1992) 42 l-426
The structure and dynamics of 1,3,5-cycloheptatriene and 1,3_cycloheptadiene radical cations in low-temperature matrices. An ESR investigation Yoshihiro Yoshihisa
Hiroshi
Kubozono, Tatsuya Matsuda. Yasuhiko
Nakamura
and Taku
Miyamoto, Gondo
Makoto
Aoyagi,
Ata’,
Matsuo
Department of Chenucal Suence and Technology. Faculty ofEngrneenng, Hakozalci, Hlgashlku. Fukuoka 812, Japan Received
Masafumi
Kyushu Unwers@v.
18April I99 I, m final form 10 October I99 I
ESR spectra of the radxal species dewed from “‘Co y-ray lrradlatlon of 1.3.5.cycloheptatrlene ( 1.3.5-CYT) and 1,3-cycloheptadlenc ( I.3-CYD) m halocarbon matrlces have been studied m the temperature range 70-I 30 K. Rmg mvewon across the molecular plane occurs m the radical catlon 1.3.5~CYT+ m CCI,CF,. the actlvatlon energy bemg 1.7 kcal/mol. Above 90 K. I .3,5-CYT+ IS deprotonated thermally m CC12FCCIF:. No dynamlcal effect has been observed for I.3-CYD+
1. Introduction ESR spectroscopic studies on the radical species radiolytically derived from cyclic hydrocarbons in halocarbon matrices have been done by many investigators [ l-l 21. Shida and co-workers studied the ring puckering of the tetrahydrofuran radical cation [ 71, while Iwasaki et al. investigated the dynamical properties of the cyclopropane and cyclohexane radical cations [ 8 1. Lund and co-workers investigated the ring puckering (pseudorotation) and inversion of the cyclopentane radical cation [ 91 and the interconversion between the two twisted CZ conformations of the cyclopentyl radical [ IO]. Recently. Shiotani and co-workers reported on the mirror inversion between two energetically equivalent structures of the radical cations of methyl-substituted cyclohexanes [ 11.121. We here report the dynamical behavior of 1.3,5CYT+ in CCl,CF3 and other halocarbon matrices in the temperature range 70- 130 K. as well as the ther’ Present address: SONY Research
Center,
Yokohama
740,
05.00 0 1992 Elsewer Sc!ence Pubhshers
B.V
ma1 reaction of it in CClZFCClF2. The molecular structure and dynamical behavior of 1.3-CYD+’ will also be discussed.
2. Experimental Commercially available I ,3,5-CYT, 1.3-CYD (Aldrich) and the halocarbons CCIJF, CC13CF3, CCl,FCClF,, and CCIZFCCllF (Tokyo Kasei) were used as received. Sample solutions of about 0.1-0.2 ~01% concentration were prepared under vacuum. The radical species were generated by ‘j°Co y-ray irradiation at 77 K at a total dose of 0.7 Mrad (1 Mrad= 10” J kg-‘). ESR measurements were made with a JEOL RE3X ESR spectrometer combined with a temperature controller (Oxford Instruments E900 helium flow cryostat ).
Japan 0301-0104/92/$
All rlghtr reserved
Y. Kubozono et al. / 1.3.5~CYTand
422
I,3-CYD radical cations
3. Results and discussion 3.1. The radlcalspecles derivedflow 1,3,5CYT The ESR spectrum of 1,3.5-CYT+’ in CCl,CF, observed at 70 K is shown in fig. la to consist of three main groups of lines. Fig 1b shows that the spectrum is well simulated with the hyperfine coupling (hfc) constants of the axial P-proton aHa =57.0 G, equatorial P-proton a”“( eq) ~46.7 G, four cr-protons aH’Z4.h(4H)=5.8 G, aH’5(2H)=0 G, and the Gaussian line width AH= 5.0 G; the terms “axial” and “equatorial” are used exaggeratedly for the convenience of discussion. The results show that 1,3,5CYT+‘ has a fixed nonplanar structure in CClXCF3 at 70 K. The P-proton couplings arise from hyperconjugation. since the SOMO of the planar structure is of n-type.
Fig. I. ESR spectrum of 1.3.5-CYT+- m CC13CF, observed at 70 K (a) and the stimulated spectrum wth the ESR parameters gwen m the text (b).
E/eV
hfcc/G _
/----.
-995.2
‘OO.O I /
.‘. -996.0
\
1,3,5-CYT
OL 50.0 I
0
Fig. 2 shows the UHF-INDO hfc constants and UHF-AM 1 total energy calculated for the geometries optimized using the single-point UHF-AM 1 calculation [ 131, as functions of r3?under the constrained C, symmetry of 0, = 0’. 8, and o2 are the dihedral angle between the planes C,-C,-C,-C, and C,-C,-C,-C,, and that between the planes C,-C,-C, and C,-C2C5-C,, respectively. The initial optimization was done by allowing both 8, and @Ito vary. In 1,3,5-CYT, 0,=40.5&2’ and &=36.5&2” as determined by electron diffraction [ 141. The 19~value of 0” for 1.3,5CYT+.. associated with the UHF-AM 1 energy minimum, shows that the carbon framework, C,-C&3-
<
_,OO~_._t_-*_ 0
IO
20
*&i
-997.0
*
:&j
30
40
50
02/ O Fg. 2. The UHF-INDO hfc constants and UHF-AM I total energles of 1.3,5-CYT+ 0: a”@(ah), 0: aHfl(eq). *: uHIb, w: uHlr. 0: a”jn, *: total energy.
C&Zs-C,, is more nearly planar in 1,3,5-CYT+’ than in 1,3,5-CYT. The UHF-INDO hfc constants a”fl( ax) and a “fl( eq) are much larger than the experimental
Y. Kubozono et al. / 1.3.5~CYTand
423
I,3-CYD radtcal cations
ones, and from the experimental and theoretical ratios of a “D(ax) to a “fl( eq) 0: is estimated to be 5 ‘. The theoretical uH1,h and Q”~.~ are practically independent of 19:and are close to each other. The UHFAM 1 total energy is minimal at &=O”. In view of these results. 1.3,5-CYT”. is slightly nonplanar in CCl,CF,. Fig. 3 shows the ESR spectra of 1.3,~~CYT+’ in CCl$ZF, observed in the range 90-120 K. every spectrum consisting of three main groups of quintet lines. The typical line width alternation occurs at 90 K and with increasing temperature the alternation diminishes gradually. This is attributable to the ring inversion illustrated below.
108 K
114 K
120 K
1,3,5-CYT+’ disappeared above 125 K before reaching the fast-motional limit, where the intensity ratio among the three main groups of lines is 1:2: 1. The ring inversion exchanges the axial and equatorial /3protons, while it affects the other protons only little. Thus, two-site jump simulatron was done with the modified Bloch equation, as fig. 4 shows. The jump rate was determined at every 2 K in the range 90- 121 K. The Arrhenius plot is linear above 104 K, as fig. 5 shows, and a least-squares fit gives an activation energy of 1.7 kcal/mol. This value is comparable with those for the ring inversions of the radical cations of tetrahydrofuran, cyclopentane and pyrolidine in halocarbon matrices [ 7,9,15 1. In the range 90-l 02 K a much smaller activation energy of 0.4 kcal/mol was obtained. Concerning the bent Arrhenius plot, at present we can only suggest a matrix phase transition around 104 K. The mechanism involved deserves to be investigated in detail. Shida and co-workers previously found a similar Arrhenius plot for the internal rotation of the methyl group in the dimethylether
Fig. 3 ESR spectra of 1,3.5-CYT’ temperature range 90- 120 K.
m CCI,CF,
observed
in the
radical cation in CC13F and attributed the smaller activation energy to tunneling [ 16 1. A spectral change due to dynamical motion was not clearly observed in the other halocarbon matrices, CC13F, CC12FCClF,, and CC12FCC12F. The tripletquintet ESR spectrum of 1,3,5-CYT+’ in CC13F observed at 90 K is the same as those at 15 and 130 K, except for the increasing line broadening with decreasing temperature. In other words, the effect of dynamical motion is not appreciable, indicating that the difference between the hfc constants of the axial and equatorial P-protons is small as compared with the line width of about 5.0 G. Thus, 1,3,5-CYT+. is more nearly planar in CCl,F than in CC13CF3. Fig. 6 shows the ESR spectrum observed at 80 K and that at 90 K for the radical species derived from I ,3,5-CYT in CClzFCClF2. The former spectrum can
424
Y. Kubozono et al. / I,J,S-CYTand
kSj.56
x10*s-’
1.3~CYD radual cations
Fig.
Arrhemus
plot of the rate constant,
In k versus
I /T.
Fig. 4. Smulatlon ofthe ESR spectra of 1,3,5-CYT+’ m CCIjCFj observed tn the temperature range 90-120 K For details, see text.
be attributed to 1,3.5-CYT+‘, while the latter is assigned to a deprotonation-type neutral radical, C,H;. Fig. 6c shows that the spectrum is well simulated with the reasonable hfc constant u” (7H) ~4.2 G for the seven cr-protons and the line width Al?= 3.3 G [ I 7 1. The 80 K spectrum of I.35CYT+ ’ is probably a little distorted by the presence of C,H;. The occurrence of thermal deprotonation reflects directly the softness of the matrix.
Fig. 6. ESR spectra. observed at 80 K (a) and 90 K (b), of the radlcak generated by the exposure of 1,3,5-CYT to “Co y-rays m CCLFCCIF, at 77 K. The respective spectra are asslgned to 1.3.5CYT+ and the deprotonatlon-type radical, C,H,. The spectrum (c) snnulated with the ESR parameters given m the text refers to . the ooserved spectrum (b)
Y. Kubozono et al. / 1.3.5CYTand
425
1.3~CYD radrcal cations
3.2. I,3-CYD+’ Fig. 7a shows the ESR spectrum of 1,3-CYD+’ in CC13CFj observed at 80 K. Fig. 7b shows that the spectrum is well simulated with the hfc constants of the two axial/?-protons a”fl(ax) =28.5 G, two equatorial D-protons aHo = 10.0 G. two a-protons aH~~5(2H)=10.0G,aH3~4(2H)=OG,aH~(2H)=OG and the line width AH=5.4 G. The observed spectrum gives no evidence for dynamical averaging even at high temperatures ( > 100 K), indicating that in the matrix the structure of 1,3-CYD+’ is practically fixed in the range 40-l 10 K; the radical disappears around 120 K. Fig. 8 shows the UHF-ND0 hfc constants and UHF-AM 1 total energy as functions of 0,. As in fig. 2, 8, is fixed at O”, where the UHF-AM1 energy is minimal in the initial optimization allowing both 0, and 8: to vary. Fig. 7. ESR spectrum of 1.3-CYD+‘ m Ccl&F3 observed K (a) and the simulated spectrum with the ESR parameters m the text (b)
at 80 gwen
Yax. Yeq. hfdG
E/eV
Oeq.
1,3-CYD
Fig. 8 The UHF-INDO hfc constants and UHF-AM1 total energles of 1,3-CYD+ A: a”#(ax), A: aHB(eq), 0: aHy(ax). 0: a”Y(eq). n : a H25 , 0: a”,+, *: total energy.
Incidentally, 8, is 0” in I,3-CYD [ 181. Just as in 1,3,5-CYT+‘, the theoretical aHg values are overestimated, and from comparison of the experimental and theoretical ratios of aHo to the aHB(eq) 8, is assessed to fall in the range 15-25 O. The other theoretical hfc constants practically agree with the observed ones in this & range. The total energy reaches a minimum at e2=00, so that tY2range can be regarded as
426
Y Kubozono et al. / 1,3,5-CYTand
consistent taking account of the matrix effect. The lower degree of planarity in 1,3-CYD+’ than in 1,3,5CYT+’ may be associated with the resistivity against ring inversion in the former. With increasing 6, the UHF-AM 1 total energy increases more steeply in 1,3CYD+’ than in 1.3.5-CYT+‘, as fig. 8 shows, which also tends to prevent ring inversion. On the other hand, 1.3-CYD+’ is more nearly planar than 1,3-CYD for which &= 72.8’ as determined by electron diffraction [ 18 1. At 90 K the spectrum for the species derived from 1.3-CYD in CC12FCC1F2 is ascribed to 1,3-CYD+‘, but it turns to be complex with increasing temperature. The spectrum in CC13F did not show any temperature dependence up to 150 K, above which 1.3-CYD+‘ disappeared.
4. Concluding remarks In the radical species studied, the 19~values oftotalenergy minima deviate slightly from 0”. probably reflecting the matrix effects. The fairly higher degree of planarity of 1.3,5-CYT+. as compared to that of 1,3CYD+’ may be associated with the occurrence of dynamical motion in the former.
Acknowledgement The authors are grateful to Professor Yusaku Ikegami of Tohoku University and Professor Masaru Shiotani of Hiroshima University for their helpful discussions. The authors also thank Dr. Hiroshi Sekiya of Kyushu University and Dr. Noriyuki Kishii of SONY Research Center for their valuable suggestions. The MOPAC Ver. 5.0 was used for the UHFAM 1 calculations [ 13,19,20]. MO calculations were
1.3~CYD radlcalcatlons
carried out on a FACOM M-780/20 computer Computer Center of Kyushu University.
at the
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